Expression of mouse mammary tumor virus genes in lymphoid cells is being investigated as a potential mechanism of subverting the host immune response to viral-induced mammary tumors. Immunofluorescence studies have revealed evidence of MMTV cross-reacting antigens on the surface of splenocytes of B-cell lineage. Our studies are designed to investigate 1) what viral proteins are expressed on the cell surface of splenocytes 2) what RNAs encode for these putative viral proteins 3) whether an endogenous and/or exogenous MMTV is responsible and 4) the immunologic alterations induced by viral proteins so encoded.